Monitoring systems of this type have been described, for example, in U.S. Pat. No. 4,193,023 issued in the names of two of us, Robert Buck and Gerd Marhofer, and in various earlier patents cited therein. A system of this nature usually includes an electronic switch, such as a thyristor, which is triggered to energize a load for signaling the occurrence of the event to be monitored. The trigger signal may be derived from the output of an oscillator whose amplitude changes, e.g. in response to the approach of a ferromagnetic element, as a result of such occurrence.
U.S. Pat. No. 4,135,124 in the name of Robert Buck, discloses the use of an Exclusive-OR gate as a means for enabling the selective emission of a high or a low output signal upon an increase or a decrease of the oscillator voltage whereby an electronic switch such as a thyristor will fire when the element being monitored either approaches or departs from the vicinity of the motion detector; with its anode tied to a supply of raw-rectified alternating voltage, the thyristor is continuously retriggered in the presence of a high voltage at the output of the Ex-OR gate so as to draw an increased load current. The Ex-OR gate, therefore, acts as a selective inverter for the control signal emitted by the motion detector.
Since the load current in such a system is either "on" or "off", depending on the conductive or nonconductive state of the electronic switch, a malfunction resulting in a prolonged absence of such load current may go undiscovered by being wrongly interpreted as either the occurrence or the nonoccurrence of the event to be detected. It is therefore desirable to provide means in a system of this nature for indicating the existence of a malfunction to an operator. For this purpose it has already been proposed to provide the system with two outputs carrying antivalent signals so that either one or the other output will conduct in any event. Under some circumstances, however, a malfunction could disable the detector while still keeping one of these outputs energized.